Discussion about the Korg PolySix synthesizer

First of all, hello and thank you for having me as a member of this group, which I've been enjoying endlessly over the last while.

I recently started restoring my PolySix, and spent last night moving all the boards into the new walnut chasis I had made.

While testing with headphones afterwards, I notice a hum which kicks in between values 2 & 5 on the speed/intensity knob with Ensemble turned on. If I turn the knob beyond 5 the hum goes away. There is also a less obvious distant crackling/ rustling noise through all values.

The hum issue is also true, but not as aggressively when Phaser is the effect selected. 

The Chorus does not have this problem.

This synth was serviced before I bought it, and the effects board doesn't seem to have any dodgy caps. the KLM367 board had been replaced, and all was well.

I'm guessing I'm only noticing this issue now that I'm using headphones, as before now I had it hooked up in the studio.

Could anybody suggest where I could start looking to solve this. Could it bee something as basic as the pot?

Thanks in advance.

Jay